Electrochemical conversion cells, commonly referred to as fuel cells, produce electrical energy by processing reactants, for example, through the oxidation and reduction of hydrogen and oxygen. Hydrogen is a very attractive fuel because it is clean and it can be used to produce electricity efficiently in a fuel cell. The automotive industry has expended significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Vehicles powered by hydrogen fuel cells would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
In many fuel cell systems, hydrogen or a hydrogen-rich gas is supplied through a flowpath to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied through a separate flowpath to the cathode side of the fuel cell. An appropriate catalyst (for example, platinum) is typically disposed to form on these respective sides an anode to facilitate hydrogen oxidation and as a cathode to facilitate oxygen reduction. From this, electric current is produced with high temperature water vapor as a reaction byproduct. In one form of fuel cell, called the proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell, an electrolyte in the form of an ionomer membrane is situated between the anode and cathode to form a membrane electrode assembly (MEA) which is further layered between diffusion layers that allow both gaseous reactant flow to and electric current flow from the MEA. The aforementioned catalyst layer may be disposed on or as part of the diffusion layer or the membrane.
To increase electrical output, individual fuel cell units are stacked with bipolar plates disposed between the diffusion layer and anode electrode of one MEA and the diffusion layer and cathode electrode of an adjacent MEA. Typically, the bipolar plates are made from an electrically-conductive material in order to form an electrical pathway between the MEA and an external electric circuit. In such a stacked configuration, the bipolar plates separating adjacently-stacked MEAs have opposing surfaces each of which include flow channels separated from one another by raised lands. The channels act as conduit to convey hydrogen and oxygen reactant streams to the respective anode and cathode of the MEA, while the lands, by virtue of their contact with the electrically conductive diffusion layer that is in turn in electrical communication with current produced at the catalyst sites, act as a transmission path for the electricity generated in the MEA. In this way, current is passed through the bipolar plate and the electrically-conductive diffusion layer.
Fuel cells convert a fuel into usable electricity via chemical reaction. A significant benefit to such an energy-producing means is that it is achieved without reliance upon combustion as an intermediate step. As such, fuel cells have several environmental advantages over internal combustion engines (ICEs) and related power-generating sources. In a typical fuel cell (such as a proton exchange membrane or polymer electrolyte membrane (in either event PEM) fuel cell), a pair of catalyzed electrodes are separated by a polysulfonated or related medium (such as Nafion™) such that an electrochemical reaction may occur when an ionized form of a reducing agent (such as hydrogen, H2) introduced through one of the electrodes (the anode) crosses the ion-transmissive medium and combines with an ionized form of an oxidizing agent (such as oxygen, O2) that has been introduced through the other electrode (the cathode). Upon combination at the cathode, the ionized hydrogen and oxygen form water. The electrons that were liberated in the ionization of the hydrogen proceed in the form of direct current (DC) to the cathode via external circuit that typically includes a load. The flow of this DC energy is the basis for power generation by the fuel cell.
Fuel cells and associated electrical systems must be protected against short circuits to prevent components and wiring from overheating and being damaged. Usually this is accomplished by using fuses, and/or circuit breakers, or other protection devices such as surge protectors. The unique short circuit characteristics of a fuel cell stack prevent passive overcurrent protection devices such as fuses and/or circuit breakers from being effective solutions. Therefore, active techniques whereby a short circuit is detected by the controls system and cleared by commanding a switching device open are necessary. It is desirable to have multiple methods of detecting short circuit events so that the failure of any single method does not result in failure of the overcurrent protection system.